(1) Field of the Invention
The present invention relates to the fabrication of integrated circuits and more particularly to a process used to fill contact/via holes of sub-half-micron in dimension.
(2) Description of Prior Art
The semiconductor integrated circuit industry is continually to micro-miniaturization i.e. to produce semiconductor devices of sub-micron and sub-half-micron features with increase performance and still lowering the cost of manufacturing. The use of sub-half-micron features allows more extremely small chips to be obtained from a specific size starting silicon wafer, thereby not only the cost of a specific chip is reduced, but the performance of the devices is also enhanced due to reduction in resistance and capacitance. However, the use of sub-half-micron features invariably increases the risk of yield and reliability failure.
In order to form an electrical interconnect/via for a multilayer semicondcutor integrated circuit, the via holes are filled with metal, such as aluminum or tungsten. The conventional aluminum deposition techniques such as sputtering or evaporation have difficulty in conformally filling via holes. Furthermore, the small diameter aluminum contact/vias can not sustain the high current density requirement, and invariably have reliability failures as a result of electromigration. Currently, the semiconductor manufacturing industry has switched to the use of tungsten for filling these sub-micron and sub-half-micron contact/via holes, on account of the improved conformality characteristics of the low-pressure chemical vapor deposited (LPCVD) tungsten. In addition, the high current carrying property of tungsten reduces the risk of electromigration failures found in aluminum filled contact/vias. However, the LPCVD tungsten tends to leave a buried void or seam in the center or middle of the contact/via upon completion of the deposition, due to the nature of the LPCVD process, i.e. it fills the via hole from the side and edge of the via. This buried void or seam in the tungsten fill is generally the result of excessive build-up of the underlying barrier layer around the top edge and along the upper portion of the wall of the via opening, thus prevent flow of source material to the center and lower portion of the via hole during subsequent metal fill deposition. Furthermore, this buried void or seam formed in the middle of the contact/via is usually exposed during the subsequent processing, such as dry etching process. The size of the seam or the void in the tungsten contact becomes larger, and creates difficult topography for subsequent metallization coverage. Many processes and techniques have been developed attempting either to patch-up the void or seam or to produce a seamless tungsten contact hole fills.
One of the conventional processes used for Tungsten via hole fill is by depositing an underlayer of electrically conductive Titanium Nitride or Tungsten nitride film which promotes nucleation of Tungsten deposition. One of the problems encountered in the deposition of these titanium nitride or tunsten nitride film by the conventional Plasma Vapor Deposition (PVD) in extremely small via openings, such as sub-micron contact/vias, where the aspect ratio, i.e. ratio of the depth of the contact hole to the diameter opening, is large, the thickness of TiN film deposited on the bottom of the hole is severely limited due to the pinch-off effect around the top opening of the hole by the uneven build-up of the TiN film around the top edge of the via opening.
To circumvent this problem, Shein-Sen M. Wang, in U.S. Pat. No. 5,187,120 has described a selective chemical vapor deposition (SCVD) method which preferentially deposits tungsten in the via opening but resists deposit of tungsten on the oxide exposed surface. This method consists of depositing a layer of a first phase of titanium nitride or tungsten nitride by either, which promotes nucleation of tungsten followed by the deposition of the oxide layer, the wall of the opening, and an overlayer of a second phase of titanium nitride or tungsten nitride which resists nucleation of tungsten on the oxide exposed surface. The SCVD process described by Wang is an extremely complicated long and costly process, it consists of first forming a TiSi.sub.x (titanium silicide) layer with predetermined electrical conductivity, followed by implantation of Nitrogen ion into the TiSi.sub.x layer to form a first phase TiN layer within the TiSi.sub.x layer, forming an oxide layer over the N implanted TiSi.sub.x layer, forming another layer of second phase TiN on the exposed oxide surface by careful sputtering of CVD on the oxide layer, etching the via opening and exposing the TiN at the bottom of the opening, rapid thermal annealing, and selectively depositing tungsten by the Silane/tungsten hexaflouride process.
This invention will describe a process which will eliminate the thick pinch-off effect of the conventional physical vapor deposition (PVD) TiN film around the top edge of the contact/via openings without the cost of deposition of two different phases of CVD (Chemical Vapor Deposition) metal nitride films described by Wang. This invention disclosed a TiN etching process which is introduced to remove the thick PCV TiN at the top and near the top of the contact/via opening without reducing the thickness of the TiN films on the wall of the lower part of the hole and the bottom of the hole. This is followed by redeposit a second TiN film, thereby, the total thickness of TiN film in the bottom and the lower wall of the via opening is equivalent to the TiN film on the top, hence a uniform barrier layer of TiN lining is created in the contact/via hole. Furthermore, the second add-on TiN layer disrupts the columnar grain of the first TiN layer. Consequently, the resulting TiN layer provides a better barrier layer and promotes more uniform nucleation sites for subsequent tungsten fill deposition.